1. Field of the Invention
This invention relates to the in-situ combustion of a subterranean, viscous oil-containing formation for the recovery of oil. More paticularly, the present invention is an in-situ combustion method for the recovery of viscous oil from subterranean, viscous oil-containing formations wherein the in-situ combustion operation is initially conducted using the injection of a mixture of oxygen and an inert gas with a predetermined low oxygen concentration, increasing the oxygen concentration to a predetermined higher concentration, and subsequently injecting a mixture of oxygen and recycled produced combustion gas enriched in carbon dioxide with a predetermined oxygen concentration so as to eliminate safety problems associated with the production of high concentrations of oxygen from the associated production wells.
2. BACKGROUND OF THE INVENTION
A variety of supplemental recovery techniques have been employed in order to increase the recovery of viscous oil from subterranean viscous oil-containing formations. These techniques include thermal recovery methods, waterflooding and miscible flooding.
Of the aforementioned recovery methods, in-situ combustion appears to be the most promising method of economically recovering large amounts of viscous hydrocarbon deposits with currently available technology. The attractiveness of the in-situ combustion method arises primarily from the fact that it requires relatively little energy necessary for sustaining combustion of the hydrocarbon deposits. In contradistinction, other in-situ techniques, such as electrical resistance heating and steam injection require considerable amounts of energy, e.g., to heat the steam at the surface before it is injected into the viscous oil-containing formation.
Conventional in-situ combustion involves drilling of at least two substantially vertical wells into the formation, the wells being separated by a horizontal distance within the formation. One of the wells is designated an injection well, and the other a production well. The recovery of oil is accomplished by raising the temperature of the in-place oil adjacent the injection well to combustion temperatures by some suitable means, e.g., with some type of a conventional down hole heater/burner apparatus, or by steam injection and then supporting combustion by injecting an oxygen-containing gas such as air, oxygen enriched air, oxygen mixed with an inert gas, or substantially pure oxygen. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain the high temperature combustion front which is formed, and to drive the front through the formation toward the production well. As the combustion front moves through the formation, it displaces ahead of it the in-place oil reduced in viscosity as well as other formation fluids such as water and also combustion gas produced during the combustion process and these fluids are recovered from the formation via the production well.
As an improvement in the in-situ combustion operation, water or steam may be injected either simultaneously or intermittently with the oxygen-containing gas to scavenge the residual heat in the formation behind the combustion and to maintain reservoir pressure, thereby increasing recovery of oil. This is sometimes referred to as wet combustion.
The use of oxygen enriched air or substantially pure oxygen for in-situ combustion operations is being seriously considered as an alternate strategy to air combustion. One of the disadvantages of this process is the danger of working with high purity oxygen in and around an oilfield environment. One danger which must be addressed is the possibility of contacting high concentrations of oxygen with the produced oil in a production well (where conditions are still hot) or in flow lines.
U.S. Pat. No. 4,042,026 to Pusch et al. discloses a method for initiating an in-situ combustion operation to eliminate the dangerous place of injecting oxygen whereby igniters are injected into the upper region of the formation and an inert gas is injected into the lower region of the formation, and thereafter an oxygen-containing gas is injected at a predetermined oxygen concentration and rate to initiate combustion, followed by increasing the oxygen concentration and/or rate of the injected gas to a maximum value.
Accordingly, it is a primary object of this invention to provide an improvement in the prior art in-situ combustion processes to eliminate the safety problem associated with the production of high concentrations of oxygen at the production wells.